1. Field of the Invention
This invention relates to electrical contact structures formed between silicon carbide and silicide-forming metals, having low specific resistances and ohmic behavior and long-term electrical and structural stability, and methods for making the same.
2. Description of the Related Art
An extensive technology of semiconductor devices has been developed based upon the properties of single crystal silicon and other similar materials which may be doped, heat treated, and otherwise processed to produce adjacent layers and regions of varying electronic characteristics. The use of devices produced by silicon technology is generally limited to operation at ambient or, at most, moderately elevated temperatures and in non-corrosive, inert atmospheres. The temperature limitation is a consequence of the small band gap of silicon (1.1. eV), which leads to large leakage currents and device failure at elevated temperatures. In addition, rapid diffusion of dopants and/or impurity species can occur in silicon, which in turn can substantially alter the character of the fabricated semiconductor device. The limitation to relatively inert environments results from the high chemical reactivity of silicon in many corrosive environments, which also can alter the character of the fabricated device. Silicon devices are also limited as to power level, frequency, and radiation tolerance by the materials used therein.
For some applications, the temperature, environmental, and other use limitations on silicon microelectronic devices may be overcome by the use of proper cooling and packaging techniques. In other applications, these limitations have prevented the use of silicon for integrated circuit technology. For example, in many spacecraft and aircraft applications, elevated temperatures are encountered, and it is not always possible to insure that adequate cooling will be provided. In high power applications, device temperatures can rise to levels which degrade or destroy the device solely through internal heating. Silicon's inability to withstand high temperatures limits the amount of power which can be generated or controlled by silicon electronics. In addition, internal thermal transients in devices otherwise operating at ambient temperature can rapidly destroy the operability of the device unless extensive cooling is provided. Such cooling requires that the device be larger in size than might otherwise be necessary, in part defeating the purpose of the integrated circuit technology.
There has therefore been an ongoing, but as yet not fully successful, search over a period of twenty years to identify and develop a semiconductor technology based in other materials. Such a technology would desirably allow the fabrication of devices for use at higher temperatures such as, for example, the range of at least about 400.degree. C. to 600.degree. C., and in applications not amenable to the use of silicon. Because corrosive effects can be greatly accelerated at elevated temperatures and pressures, any such materials and devices must also exhibit excellent corrosion resistance at the elevated use temperatures and over a range of pressures from vacuum to many atmospheres. Some generally desirable characteristics of such materials have been identified, including large band gap, good electrical conductivity, high electric field breakdown strength, low dielectric constant, ability to be doped to produce regions of varying electronic characteristics, a high melting temperature, good strength at operating temperatures, resistance to diffusion by undesired foreign atoms, good thermal conductivity, thermal stability, chemical inertness, and the ability to form stable ohmic and rectifying external contacts.
Silicon carbide has been identified as a candidate material meeting the indicated requirements. Silicon carbide has a high decomposition temperature, good strength, good resistance to radiation damage, and good corrosion resistance in many environments. Silicon carbide has a high breakdown field strength, ten times that of silicon, a relatively large band gap, low dielectric constant, and a thermal conductivity of more than three times that of silicon at ambient temperature. The diffusion coefficients in silicon carbide are also much smaller than those in silicon or gallium arsenide, and so silicon carbide is resistant to the diffusion of impurity species. Silicon carbide may be processed by several techniques similar to those used in silicon device technology, and in many instances silicon carbide devices may be substituted at moderate and low temperatures for silicon devices. Silicon carbide semiconductor device technology therefore offers the opportunity for supplementing, and in some instances replacing, conventional silicon device technology.
Silicon carbide crystallizes in a cubic polytype as well as numerous hexagonal and rhombohedral polytypes. The 6H-SiC hexagonal polytype is currently the most developed for electronic device applications, but other polytypes, especially the hexagonal 4H-SiC and the cubic 3C-SiC, have also attracted attention for use in high power, high temperature devices. Properties of the 6H polytype are compared with those of silicon and gallium arsenide below. Combined, these advantages in physical and electrical properties predict high maximum acceptable junction temperatures and will allow extremely high power and high temperature operation.
______________________________________ Property Si GaAs 6H-SiC ______________________________________ Eg (eV) 300K 1.11 1.43 2.93 Electron mobility (R.T. cm.sup.2 /V-s) 1400 8500 400 Hole mobility (R.T. cm.sup.2 /V-s) 600 400 40 Breakdown V (E.sub.b, 10.sup.6 V/cm) 0.3 0.4 4.0 Thermal conductivity (W/cm-.degree.C.) 1.5 0.5 5.0 Saturated electron drift velocity (10.sup.7 cm/s) 1.0 2.0 2.0 Dielectric constant 11.8 12.8 10.0 Maximum junction temp (.degree.C.) .apprxeq.200 .apprxeq.300 .apprxeq.1000 ______________________________________
Silicon carbide in single crystalline form may be prepared by sublimation growth. Silicon carbide epitaxial layers may be formed or deposited by many techniques, including reactive evaporation and chemical vapor deposition (CVD). CVD is a particularly desirable fabrication approach, as it permits the controlled growth of undoped and doped layers and structures of a variety of forms. The layers may be doped in situ by inclusion of dopant species in the source gases of the CVD process, or after growth by implantation. Thus many of the processes for silicon carbide device fabrication are well-known.
Ohmic contacts, important elements of any device, are critical to the performance and stability of high power and high temperature devices. Wide band gap materials, including silicon carbide (SiC), diamond, and the III-V nitrides, currently suffer from a dearth of suitable contact strategies. Contact strategies to date have been fairly conventional, involving implantation, metallization, and deposition of simple silicides directly onto the silicon carbide surface. Although silicon carbide devices have been operated at temperatures as high as 650.degree. C. (J. W. Palmour, H. S. Kong and R. F. Davis, Appl. Phys. Lett. 51 (2028 (1987), the contacts have not demonstrated contact resistances comparably low to those achieved in gallium arsenide or silicon, or long term stability.
In addition to the semiconductor material, the entire microelectronic device and package (including metallization) must be suitable for high temperature operation. Stability of the contact is extremely important. Typical projected junction operating temperatures for these devices are as high as 1000.degree. C. High temperature operation exacerbates diffusion; contacts must be designed to remain stable, both structurally and electrically, over the expected life of the device.
In addition to being stable, the contact resistance must be very low. While low contact resistances are desirable in all devices, they are a necessity in high power devices. A high contact resistance will increase the forward voltage drop and thus severely decrease device efficiency. Specific examples of high temperature devices that would desirably be fabricated in silicon carbide include power MOSFETs, thyristors, rectifiers and high frequency devices such as IMPATTs. Today, typical specific contact resistances to SiC are in the range of mid 10.sup.-5 to 10.sup.-4 .OMEGA.-cm.sup.2. Tomorrow, specific contact resistances need to be in the 10.sup.-6 .OMEGA.-cm.sup.2 range, similar to those attained in GaAs-based materials. Without low resistance contacts, the GaAs devices would not work; an analogous fate will befall SiC devices if low specific contact resistance strategies are not deployed.
A number of metals have been investigated for ohmic contacts to SiC. Typical ohmic contacts to SiC are nickel for n-type contacts and aluminum for p-type contacts. Waldrop and Grant (Appl. Phys. Lett. 56 (1990) 557) have examined additional metals in the as-deposited state; Pd, Au, Co, Ti, Ag and Tb. From the I-V characteristics, Pd, Au and Co are rectifying whereas Ti, Ag, Tb and Al are "ohmic-like" (essentially linear I-V curve in forward and reverse bias). However, as-deposited metals are not suitable for high temperature contacts--they must be annealed at a temperature higher than their intended operating temperature. During annealing, these metals chemically react with the SiC to produce metal carbides and/or metal silicides, which are stable at the device operating temperatures.
Direct silicide formation on SiC is complicated by the fact that SiC, in addition to silicon, also contains carbon. After annealing, complicated metallurgy at the interface results. For example, annealing Ti on SiC at temperatures between 570-800.degree. C. resulted in diffusion of C and Si into the titanium and the formation of TiC and excess silicon (M. B. Chamberlain, Thin Solid Films 72 (1980) 305). No titanium silicides were observed. While TiC is a potential contact, the silicide would be preferred because of its lower sheet resistance. In addition, carbide formation continued for at least 20 hrs at 606.degree. C.; clearly not acceptable metallurgy for a stable high temperature contact. Other examples include Mo and Pt on SiC. In the case of Mo, a Mo.sub.2 C/Mo.sub.5 Si.sub.3 /SiC multilayer was observed after annealing (S. Hara et al., Jap. J. Appl. Phys. 29 (1990) L394). Annealing Pt on SiC resulted in a periodic structure consisting of alternating layers of platinum silicides and carbon (T. C. Chou, J. Mater. Res., 5 (1990) 601).
We have demonstrated Ti-based contacts to SiC with specific contact resistances as low as 10.sup.-4 .OMEGA.-cm.sup.2, as well as co-evaporated TiSi contacts which showed resistances as low as .about.5.times.10.sup.-3 .OMEGA.-cm.sup.2. These results indicate that co-evaporation is not ideal for deposition of TiSi to produce low resistance contacts. From Chamberlain's results discussed above, it is likely that the actual contact in the Ti case was TiC. More recent results indicate that Ti.sub.5 Si.sub.3, as well as TiC, form at the interface between silicon carbide and titanium (L. B. Rowland et al., ONR Workshop on SiC Materials and Devices, Sep. 10-11, 1992, Charlottesville, Va. pp. 8). This work used epitaxially deposited Ti, followed by 700.degree. C. annealing in vacuum.
Tungsten and tungsten-gold alloys, after annealing at temperatures between 800-1600.degree. C., had contact resistances between 7.times.10.sup.-4 and 7.times.10.sup.-5 .OMEGA.-cm.sup.2, depending on the SiC carrier concentration (M. Anikin et al., Ioffe Physico-Technical Institute, Academy of Sciences of the USSR, Leningrad). Sputtered and alloyed TaSi.sub.2 has been used in devices operating up to 650.degree. C., although no contact resistances or stability data have been reported (J. W. Palmour, H. S. Kong and R. F. Davis, J. Appl. Phys. 64 (1988) 2168). Nickel has also been used in high temperature devices, but again no data specific to the contacts has been reported (R. F. Davis et al., Proc. IEEE. 79 (1991) 677). It is likely that in this case Ni.sub.2 Si was formed, as was reported when nickel was annealed at 600.degree. C. on SiC (I. Ohdomari, et al., J. Appl. Phys. 62 (1987) 3747). No carbides were detected, but carbon was distributed uniformly throughout the reacted film. Investigations of platinum silicide indicate that it forms a Schottky contact to SiC (N. A. Papanicolaou et al., J. Appl. Phys. 65 (1989) 3526).
Therefore there is a need for a technology capable of forming stable, low specific resistance contacts to silicon carbide in all of its useful polytypes. Key to making such stable contacts is to understand and control the metallurgy and band structure at the contact interface. In an ohmic contact, the current flow is proportional to the applied voltage, both in sign and magnitude. For a low resistance contact, large currents should be supported without a significant voltage drop across the contact region. Tunneling processes exhibit linearity over a larger range than thermionic or thermionic field emission and thus it is desired that current conduction be dominated by tunneling in a low resistance contact. A large conductance (or small specific contact resistance) is achieved with a heavily doped semiconductor and a small potential energy barrier height. Heavy doping decreases the barrier width, resulting in increased current flow and a lower potential drop across the contact. The depletion region under the contact for heavily doped material is very small, on the order of 100 .ANG. for a carrier concentration of 10.sup.19 cm.sup.-3. This means that the heavily doped contact region can be extremely thin. A small barrier height at the contact interfaces facilitates current flow with little resistive loss. The barrier height in the ideal case is determined by the work function of the metal and the electron affinity of the semiconductor. However, because of interface states, the actual barrier height often does not agree with this value.
J. M. Andrews and J. C. Phillips (Phys. Rev. Lett. 35 (1975) 56) correlated the barrier height of a number of silicide-silicon interfaces with the heats of formation of the silicides. Their work clearly showed that those metals which are more reactive with silicon, i.e. form silicides with higher heats of formation, made better ohmic contacts (lower barrier heights). From these results, silicides of Ni, Co, W, Ta, Ti, Cr, Mo and Zr have been identified as good candidate metals for formation of ohmic contacts to SiC.
One problem observed with all of these contacts is that many phases of metal silicides and carbides are possible. This is because both silicon and carbon are present in SiC. The case is much simpler for contacts to an elemental semiconductor such as silicon or germanium since only one element is available for reaction with the metal.
Accordingly, it is an object of the present invention to provide stable, low resistance ohmic contacts to silicon carbide for use in high temperature, high power devices by controlling the metallurgy of the contact interface through the use of a sacrificial interlayer structure. This interlayer presents only one element to the metal, which greatly simplifies the contact metallurgy. A further object of the invention is to provide methods for fabricating these contact structures.
Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.